Rechargeable batteries are used in a variety of industrial commercial and military applications such as fork lifts, uninterruptible power supplies, electric vehicles and military weapons systems.
Rechargeable lead-acid batteries are a useful power source for starter motors for internal combustion engines. However, their low energy density (about 30 wh/kg) and their inability to perform at high temperature, makes them an impractical power source for electric vehicles (EV), hybrid electric vehicles (HEV) and other systems requiring a high energy density source. Electric vehicles using lead-acid batteries have a short range before requiring recharge, require about 6 to 12 hours to recharge and contain toxic materials. In addition, electric vehicles using lead-acid batteries have sluggish acceleration, poor tolerance to deep discharge, and a battery lifetime of only about 20,000 miles.
Nickel-metal hydride batteries (“Ni-MH batteries”) are far superior to lead-acid batteries, and Ni-MH batteries are currently used in electric vehicles, hybrid vehicles and other forms of vehicular propulsion. For example, Ni-MH batteries, such as those described in U.S. Pat. No. 5,277,999, the disclosure of which is hereby incorporated herein by reference, have a much higher energy density than lead-acid batteries, can power an electric vehicle over 250 miles before requiring recharge, can be recharged in 30 minutes, and contain no toxic materials.
Extensive research has been conducted in the past into improving the electrochemical aspects of the power and charge capacity of Ni-MH batteries, which is discussed in detail in U.S. Pat. Nos. 5,096,667, 5,104,617, 5,238,756 and 5,277,999, the contents of which are all hereby incorporated herein by reference.
The mechanical and thermal aspects of the performance of Ni-MH batteries have important aspects of operation. For example, in electric vehicles and in hybrid vehicles, the weight of the batteries is a significant factor. For this reason, reducing the weight of individual batteries is a significant consideration in designing batteries for electric and hybrid vehicles. Battery weight should be reduced while still affording the necessary mechanical requirements of the battery (i.e. ease of transport, ruggedness, structural integrity, etc.).
Electric vehicle and hybrid vehicle applications include a critical requirement for thermal management. Individual electrode stacks are placed together in close proximity and many stacks are electrically coupled together. Therefore, since there is an inherent tendency to generate significant heat during charge and discharge, a workable battery design for electric and hybrid vehicles is judged by whether or not the generated heat is sufficiently controlled. Sources of heat are primarily twofold. First, ambient heat due to the operation of the vehicle in hot climates; second, resistive or I2R heating known as and hereinafter referred to as “joule heating” on charge and discharge, where I represents the current flowing into or out of the battery and R is the resistance of the battery.
Batteries have been developed which reduce the overall weight thereof and incorporate the necessary thermal management needed for successful operation in electric and hybrid vehicles and other applications, without reducing its energy storage capacity or power output. One such battery design is a monoblock battery. Monoblocks are multicavity packaging embodiments in which the cell cavities are all contained within one enclosure. An example of a monoblock battery is provided in U.S. Pat. No. 6,255,051 issued to Corrigan et al. on Jul. 3, 2001, the contents of which are hereby incorporated herein by reference. Another example of a monoblock battery is provided in U.S. Pat. No. 6,689,510 issued to Gow et al. on Feb. 10, 2004, the contents of which are hereby incorporated herein by reference. Another example of a monoblock battery is provided in U.S. patent application Ser. No. 09/861,914, now U.S. Pat. No. 7,264,901 issued to Gow et al. on Sep. 4, 2007, the disclosure of which is hereby incorporated herein by reference.
Polymers are widely used as materials of choice in prismatic battery enclosures due to advantages including lower cost, lower weight and easier manufacturability when compared to metal enclosures. In order to ensure that such a battery fulfills life expectations it is important to transfer heat away from the battery. Although polymers typically have excellent volume resistivity and dielectric properties, poor thermal conductivity is a drawback. Currently, there exists a need in the art for battery case having a design that may be easily modified for a plurality of applications and provide effective thermal management and mechanical stability.
FIG. 1A illustrates a battery of the prior art with an electrode stack 76 exploded from a case 10 and cover 11. The prior art electrode stack 76 comprises two half electrode stacks that are composed of a plurality of individual electrodes wherein electrode tabs 52 are connected using a terminal 80 and connection means 85. Reference H1 shows that electrode tabs of the prior art electrode stack are not compact and occupy maximum space within a battery. Reference H1 also identifies the height or vertical space occupied by the connection means 85. The connection means 85 is internal to the hermetically sealed portion of the battery. The connection means 85 passes through the case wall and makes the electrical path to the external terminal. The connection means 85 also gathers together and connects the plurality of electrodes of the electrode stack 76 to the terminal 80.
Non-limiting examples of connection means 85 include bolts, screws, nuts, rivets, welds, solder, wiring, copper blocks, aluminum blocks, plates, crimps, folds and so forth. In the prior art, electrode tabs 52 are pinched vertically and clamped to a block of the connection means 85, see FIG. 1B. The distance from the power generating electrode active area 54 of each electrode to the point of connection of the electrode tabs 52 to terminal 80 varies. The path length to terminal 80 of the outer most electrodes traverse a greater distance than the inner most electrodes. Accordingly, the electrical resistance paths for the electrodes are unequal and, as a result, current drawn from each electrode in the parallel connection is unequal across all electrodes.
The space consumed by the connection means 85 of the prior art is represented by the vertical space H1. The volume occupied within the case 10 by the connection means 85, and approximated by the height H1 and the internal horizontal dimensions of the case, is a volume not occupied by the power producing elements of the electrode stack, the electrode active area 54. The electrode active area 54 is that part of each electrode having a coating and being wetted by electrolyte. The connection means 85 necessarily gathers and connects the uncoated electrode tabs 52 to form an electrical circuit connection to the terminal 80 and the external terminals of the battery. Thus, to the extent that the volume occupied by the connection means 85 can be minimized, the volume of the power producing elements 54 of the electrode stack can be maximized. In the prior art, because of the large connection means 85, which occupy on the order of 17-20% of the cell volume, the power producing elements 54, the electrode active area, of the electrode stack are smaller. The remaining unoccupied space in the case 10 is filled with electrolytes. The vertical space H1 occupied by the connection means 85 does nothing to contribute to providing power and instead prevents filling the battery with larger electrodes.
The excessive vertical space or height H1 occupied by connection means 85 and similar connection means is a problem recognized in the prior art. Specifically, issues involving connecting electrodes are known industry-wide problems that have yet to be addressed. Accordingly, it is an object of the present invention to fill the internal battery space with larger battery cells or electrodes forming electrode stacks. It is an object of the invention to maximize internal battery space with power producing electrode stack elements. It is also an object of the invention to use connection means that occupy less vertical space within a battery. It is also an object of the present invention to equalize the electrical resistance path for electrodes. Furthermore, it is an object of the present invention to equalize current flow through each electrode in the parallel terminal connection across all electrodes. Furthermore, it is an object of the invention to have low resistance, compact connections resulting in a battery with low internal resistance.
The present invention overcomes deficiencies in the prior art by providing solutions to problems cited above as well as other problems.